Polyethylene Pipe Fitting Resins

ABSTRACT

A polyethylene resin suitable for formation into pipes or pipe fittings is disclosed, having an M15 of from 0.40 to 0.70 g/10 min. and comprising from 47 to 52 wt % of a low molecular weight polyethylene fraction, and from 48 to 53 wt % of a high molecular weight polyethylene fraction, wherein the high molecular weight polyethylene fraction comprises a copolymer of ethylene and 1-hexen or 1-octene.

The present invention relates to polyethylene resins, more particularlythose suitable for use as pipes, pipe attachments or fittings, and to aprocess for producing such resins. The present invention also relates tothe use of polyethylene compounds comprising such resins for themanufacture of pipes or pipe fittings, and to such fittings themselves.

Polyolefins such as polyethylenes which have high molecular weightgenerally have improved mechanical properties over their lower molecularweight counterparts. However, high molecular weight polyolefins can bedifficult to process and can be costly to produce.

For many HDPE applications, polyethylene with enhanced toughness,strength and environmental stress cracking resistance (ESCR) isimportant. These enhanced properties are more readily attainable withhigh molecular weight polyethylene. However, as the molecular weight ofthe polymer increases, the processibility of the resin decreases. Byproviding a polymer with a broad or bimodal molecular weightdistribution (MWD), the desired properties that are characteristic ofhigh molecular weight resins are retained while processability,particularly extrudability, is improved.

Polyethylene resins are known for the production of pipes and fittings.Pipe resins require high stiffness (creep rupture strength), combinedwith a high resistance against slow crack growth as well as resistanceto crack propagation yielding impact toughness. However, there is theneed to improve the creep rupture strength of currently available piperesins, keeping the resistance against slow crack growth and the rapidcrack propagation at least at a constant level. This would permit anincrease in the pressure rating of such pipes.

Polyethylene pipes are widely used as they are lightweight and can beeasily assembled by fusion welding. Polyethylene pipes also have a goodflexibility and impact resistance, and are corrosion free. Unlesspolyethylene pipes are reinforced, they are however limited in theirhydrostatic resistance by the inherent low yield strength ofpolyethylene. It is generally accepted that the higher the density ofthe polyethylene, the higher will be the long term hydrostatic strength.Pipe resins are known in the art which are referred to by the names “PE80” and “PE 100”. This classification is described in ISO 9080 and ISO12162. These are polyethylene resins which when used for the formationof pipes of specific dimensions, survive a long term pressure test atdifferent temperatures for a period of 5,000 hours. Extrapolationaccording to ISO 9080 shows that they have an extrapolated 20° C./50years stress at a lower prediction level (97.5% confidence level—“LPL”)of at least 8 and 10 MPa; such resins are known as PE80 and PE 100resins respectively. The density of the current basic powder used in theproduction of a PE100 compound is close to 0.950 g/cm³ (typically from0.949 to 0.951 g/cm³). Such polyethylene resins containing conventionalamounts of black pigments have densities from about 0.958 to 0.960g/cm³. Pipe fittings are also required to have such properties. Inaddition, it is desired that pipe fittings have good resistance tosagging.

It is known in the art that the key components for a good PE 100 resinare the combination of a low molecular weight high density polyethylenewith little or no short chain branching (SCB) due to comonomerincorporation and a linear low density polyethylene (LLDPE) resin withhigh molecular weight and SCB.

Usually, polyethylene resins of such composition are produced in acascade reactor process using Ziegler-Natta catalysts. Another variantmight be to blend different polyethylene powders and extrude them toform a physical blend, as opposed to a chemical blend produced using acascade reactor. However, physical blending often leads to a poor mixingof the melts, which leaves large high molecular weight microscopicparticles (referred to as gels in the art) embedded in the finalproduct. The weight fraction of the LLDPE resin is around 50% of theblend. The low molecular weight high density polyethylene (HDPE) confersa high crystallinity, and thus a high rigidity and resistance to creepfor the blend, and depresses the melt viscosity of the blend. The highmolecular weight LLDPE provides the polyethylene blend with a highdensity of tie-molecules as a result of the short chain branching, whichare responsible for the enhanced environmental stress crack growthresistance (ESCR) observed in these blends.

WO 00/60001 discloses a high density multimodal polyethylene for use inpipes and having a PE 80 and PE 100 rating, wherein the high molecularweight fraction typically comprises 45-55wt % of the composition, andhas a density less than 0.930 g/cm³ and an HLMI of less than 0.30 g/10min. The MI₅ of the resin is typically 0.2-0.3 g/10 min.

WO 02/34829 discloses a high density multimodal polyethylene for use inpipes or fittings, wherein the resin typically has an MI₅ of 0.2-0.6g/10 min, and the low molecular weight fraction (LMW block) comprises atleast 51 wt % of the resin. In Example 1 for example, the LMW blockcomprises 55 wt % of the resin, and the resin has an MI₅ of 0.63 g/10min. This resin would not be suitable for pipe fittings because the verynarrow molecular weight distribution (MWD) of the individual blocks andthe narrow MWD of the resulting composition would cause both poorhomogeneity and poor processability.

WO 02/102891 discloses a high density multimodal polyethylene for use inpipes or fittings, wherein the resin typically has an MI₅ of 0.15-2 g/10min, and the low molecular weight fraction comprises at least 53 wt % ofthe resin. In Example 10 for example, the LMW block comprises 58.4 wt %of the resin, and the resin has an MI₅ of 0.54 g/10 min. This resinwould not be suitable for pipe fittings because the very high proportionof first block would cause poor product homogeneity resulting in poormechanical properties.

JP 2000-109521A discloses a high density multimodal polyethylene for usein pipes or fittings, wherein the resin typically has an MI₅ of0.25-0.50 g/10 min, and the low molecular weight fraction comprises45-60 wt % of the resin. There is no specific disclosure of the use of1-hexene or 1-octene as comonomer in the high molecular weight fraction.The single Examples of the invention has an MI₅ of 0.42 g/10 min, butuses 1-butene as the comonomer and hence has a density of only 952kg/m³.

We have found that by selecting a particular combination of propertiesit is possible to obtain resins with the hydrostatic properties requiredfor pipes, which makes them particularly suitable for pipe fittings aswell as pipes.

Accordingly, the present invention provides in a first aspect apolyethylene resin having an MI₅ of from 0.40 to 0.70 g/10 min, andcomprising from 47 to 52 wt % of a low molecular weight polyethylenefraction, and from 48 to 53 wt % of a high molecular weight polyethylenefraction, wherein the high molecular weight polyethylene fractioncomprises a copolymer of ethylene and either 1-hexene or 1-octene.

We have found that the above choice of molecular weight, density and MI₅can lead to a marked improvement of the processing behaviour of fittingsinjections, compared with other equivalent PE100 grades, whilstmaintaining or even improving mechanical properties, such as stresscrack resistance. In particular, injection-moulded pipe fittings madeusing the resin of the invention show excellent resistance to relaxationafter injection. Such a balance of properties is not possible with aresin containing 1-butene as comonomer.

Preferably the low molecular weight polyethylene fraction comprises apolyethylene having a density of at least 0.965 g/cm³ and an MI₂ of from5 to 1000 g/10 min, and the high molecular weight polyethylene fractionhas a density of from 0.910 to 0.940 g/cm³ and an MI₅ of from 0.01 to 2g/10 min.

The present invention further provides the use of such a polyethyleneresin for the manufacture of pipes and fittings, and in a further aspecta pipe or fitting comprising the polyethylene resin of the invention.

Preferably the high molecular weight fraction comprises from 0.1-10 wt %of comonomer (ie 1-hexene or 1-octene).

Preferably the MI₅ of the resin is from 0.45 to 0.65 g/10 min. Thisvalue refers to the resin after granulation, not immediately uponexiting the polymerisation reactor. For the purposes of the presentinvention, MI₂ and MI₅ represent the fluidity indices measured accordingto ASTM standard D 1238 (1986) at a temperature of 190° C. under a loadof 2.16 kg and 5 kg respectively. Fluidity index HLMI or MI₂₁ means thefluidity index measured according to ASTM standard D 1238 (1986) at atemperature of 190° C. under a load of 21.6 kg.

The resin of the present invention preferably comprises from 48 to 51%by weight of the low molecular weight fraction, more preferably between49 and 51 wt %.

The resins according to the invention preferably have a density aftergranulation, (not immediately upon exiting the polymerisation reactor)measured according to ASTM standard D 792 (on a sample preparedaccording to ASTM standard D 1928 Procedure C) of at least 957 kg/m³,more particularly at least 959 kg/m³. Preferably the density does notexceed 963 kg/m³. Particularly preferred are resins whose density isbetween 959 and 961 kg/m³. These densities correspond to densities of“natural” resins, that is resins upon exiting the reactor before theaddition of any additives such as pigments, of at least 948 kg/m³, moreparticularly at least 950 kg/m³, preferably not greater than 954 kg/m³,and particularly between 950 and 952 kg/m³.

The density of the polymer (A) present in the resins according to theinvention is preferably at least 972 kg/m³. The density of copolymer (B)is preferably at least 910 kg/m³. The density of copolymer (B) does notexceed preferably 928 kg/m³; more particularly not 926 kg/m³.

For the purposes of the present invention, ethylene polymer (A) is anethylene polymer comprising monomer units derived from ethylene andpossibly monomer units derived from other olefins. Copolymer (B) is acopolymer comprising monomer units derived from ethylene and monomerunits derived from 1-hexene or 1-octene.

For the purposes of the present invention, hexene or octene content ismeasured by RMN¹³C according to the method described in J. C. RANDALL,JMS—REV.MACROMOL.CHEM.PHYS., C29(2&3), p. 201-317 (1989). For examplethe content of units derived from the comonomer is calculated frommeasurements of the integrals of the characteristic spectral lines ofthe comonomer (eg for hexene 23.4; 34.9 and 38.1 ppm) with respect tothe integral of the characteristic spectral line of the units derivedfrom ethylene (30 ppm).

The content in copolymer (B) of monomer units derived from 1-hexene or1-octene, hereinafter called comonomer content, is generally at least0.4 mole %, in particular at least 0.6 mole %. The comonomer content ofcopolymer (B) is usually at most 1.8 mole %, preferably at most 1.5 mole%. Particularly preferred is a comonomer content that does between 0.7and 1.1 mole %.

Ethylene polymer (A) may optionally contain monomer units derived fromanother olefin. Ethylene polymer (A) comprises preferably at least 99.5mole %, more particularly at least 99.8, mole % of monomer units derivedfrom ethylene. Particularly preferred is an ethylene homopolymer.

Polymer (A) according to the invention preferably has an MI₂ of at least200, preferably at least 250 g/10 min. The MI₂ of polymer (A) generallydoes not exceed 1000 g/10 min, preferably no more than 700 g/10 min.Polymer (A) preferably has an HLMI of at least 1000 g/10 min.

Polymer (A) preferably has an inherent viscosity ηA (measured intetrahydronaphthalene at 160° C., at a concentration of 1 g/l, by meansof an Ostwald type viscosimeter (K2/K1 approximately 620)) of at least0.45 dl/g, preferably at least 0.50 dl/g. Its inherent viscositygenerally does not exceed 0.75 dl/g, and is preferably no higher than0.65 dl/g.

The melt index MI₅ of copolymer (B) according to the invention ispreferably at least 0.005 g/10 min. It preferably does not exceed 0.1g/10 min. Copolymer (B) presents advantageously an HLMI of at least 0.05g/10 min which also does not exceed 2 g/10 min.

Copolymer (B) has in general an inherent viscosity η_(B) of at least 2.7dl/g, preferably at least 3.9 dl/g. Its inherent viscosity η_(B) doesnot exceed in general 10.9 dl/g, preferably not 7.6 dl/g.

In the resins according to the invention, the ratio between the inherentviscosity of copolymer (B) (η_(B)) and that of polymer (A) (η_(A)) isgenerally at least 4, preferably at least 6. The η_(B)/η_(A) ratio doesnot generally exceed 15, preferably not 12.

The resin according to the invention typically has an HLMI/MI₅ ratiogreater than 20, preferably greater than 25. The HLMI/MI₅ ratio does notusually exceed 150. Preferably, the HLMI/MI₅ ratio does not exceed 70.The HLMI/MI₅ ratio of the resin illustrates the broad or bimodalmolecular weight distribution of the resin.

Polymer (A) and copolymer (B) utilised in the resin according to theinvention each have a molecular weight distribution characterised by aratio M_(W)/M_(n) greater than 4. The M_(W)/M_(n) ratio means the ratiobetween the mean molecular mass by weight M_(W) and the mean molecularmass by number M_(n) of the polymer as they are measured by stericexclusion chromatography (SEC) according to the developing standardsISO/DIS 16014-1 and ISO/DIS 16014-2. The SEC is conducted in1,2,4-trichlorobenzene at 135° C. and 1 ml/min on a Waters 150 Cchromatograph equipped with a detector by refractometry. The injectionsare effected on a set of four WATERS® HT-6E columns in the followingconditions: injection of 400 μl of a 0.5 g/l solution of polymer and ofIRGANOX® 1010, linear calibration curve based on Mark-Houwink'scoefficients for polystyrenes of K=1.21×10⁻⁴ and a=0.707 and forpolyethylenes K=3.92×10⁻⁴ and a=0.725.

Polymer (A) preferably has a molecular weight distribution M_(W)/M_(n)that does not exceed 12, more particularly not 10. Copolymer (B)preferably has a molecular weight distribution M_(w)/M_(n) of at least 6but no more than 15, preferably no more than 12. It has been found thatutilisation of polymers (A) and (B) having a molecular weightdistribution M_(W)/M_(n) greater than 4 makes it possible to obtaincompositions having better homogeneity when the composition is utilisedsubsequently in comparison with compositions having the same compositionand the same characteristics but comprising ethylene polymers having amolecular mass distribution M_(W)/M_(n) below 4.

The resins of the invention are preferably obtained by means of aprocess utilising at least two polymerisation reactors connected inseries, according to which process:

in a first reactor, ethylene is polymerised in suspension in a mediumcomprising a diluent, hydrogen, a catalyst based on a transition metaland a cocatalyst so as to form from 47 to 52% by weight with respect tothe total weight of the composition of ethylene polymer (A),

said medium comprising polymer (A) in addition is drawn off from saidreactor and is subjected to expansion so as to degas at least part ofthe hydrogen, after which

said at least partially degassed medium comprising polymer (A) and someethylene and 1-hexene or 1-octene and optionally at least anotheralpha-olefin containing from 4 to 10 carbon atoms are introduced into afurther reactor in which polymerisation in suspension is effected inorder to form from 48 to 53% by weight with respect to the total weightof the composition of ethylene copolymer (B).

Polymerisation in suspension means polymerisation in a diluent which isin the liquid state in the polymerisation conditions (temperature,pressure) used, these polymerisation conditions or the diluent beingsuch that at least 50% by weight (preferably at least 70%) of thepolymer formed is insoluble in said diluent.

The diluent used in this polymerisation process is usually a hydrocarbondiluent, inert to the catalyst, to the cocatalyst and to the polymerformed, such for example as a linear or branched alkane or acycloalkane, having from 3 to 8 carbon atoms, such as hexane orisobutane.

The quantity of hydrogen introduced into the first reactor is in generalset so as to obtain, in the diluent, a molar ratio between hydrogen andethylene of 0.05 to 1. In the first reactor, this molar ratio ispreferably at least 0.1.

The medium drawn off from the first reactor comprising in addition thepolymer (A) is subjected to expansion so as to eliminate (degas) atleast part of the hydrogen. The expansion is advantageously effected ata temperature below or equal to the polymerisation temperature in thefirst reactor. The temperature at which the expansion is effected isusually greater than 20° C., it is preferably at least 40° C. Thepressure at which the expansion is carried out is below the pressure inthe first reactor. The expansion pressure is preferably below 1.5 MPa.The expansion pressure is usually at least 0.1 MPa. The quantity ofhydrogen still present in the at least partially degassed medium isgenerally below 1% by weight of the quantity of hydrogen initiallypresent in the medium drawn off from the first polymerisation reactor,this quantity is preferably below 0.5%. The quantity of hydrogen presentin the partially degassed medium introduced into the furtherpolymerisation reactor is therefore low or even nil. The further reactoris preferably also supplied with hydrogen. The quantity of hydrogenintroduced into the further reactor is in general set so as to obtain,in the diluent, a molar ratio between hydrogen and ethylene of 0.001 to0.1. In this further reactor, this molar ratio is preferably at least0.004. It does not exceed preferably 0.05. In the process according tothe invention, the ratio between the hydrogen concentration in thediluent in the first reactor and that in the further polymerisationreactor is usually at least 20, preferably at least 30. Particularlypreferred is a ratio of concentrations of at least 40. This ratio doesnot usually exceed 300, preferably not 200.

The quantity of 1-hexene or 1-octene introduced into the furtherpolymerisation reactor is such that in the further reactor thecomonomer/ethylene molar ratio in the diluent is preferably at least0.05, more preferably at least 0.1. The comonomer/ethylene molar ratiopreferably does not exceed 3, more preferably not 2.8.

The catalyst employed in the polymerisation process may be anycatalyst(s) suitable for preparing the low and high density fractions.Preferably, the same catalyst produces both the high and low molecularweight fractions. For example, the catalyst may be a chromium catalyst,a Ziegler-Natta catalyst, or a metallocene catalyst. Preferably thecatalyst is a Ziegler-Natta catalyst.

In the case of a Ziegler-Natta catalyst, the catalyst used comprises atleast one transition metal. Transition metal means a metal of groups 4,5 or 6 of the Periodic Table of elements (CRC Handbook of Chemistry andPhysics, 75th edition, 1994-95). The transition metal is preferablytitanium and/or zirconium. A catalyst comprising not only the transitionmetal but also magnesium is preferably utilised. Good results have beenobtained with catalysts comprising:

from 10 to 30%, preferably from 15 to 20%, more preferably 16 to 18% byweight of transition metal,

from 0.5 to 20%, preferably from 1 to 10%, more preferably 4 to 5% byweight of magnesium,

from 20 to 60%, preferably from 30 to 50%, more preferably 40 to 45% byweight of halogen, such as chlorine,

from 0.1 to 10%, preferably from 0.5 to 5%, more preferably 2 to 3% byweight of aluminium;

the balance generally consisting of elements arising from products usedfor their manufacture, such as carbon, hydrogen and oxygen. Thesecatalysts are preferably obtained by coprecipitation of at least onetransition metal composition and a magnesium composition by means of ahalogenated organoaluminium composition. Such catalysts are known, theyhave notably been described in patents U.S. Pat. No. 3,901,863, U.S.Pat. No. 4,2942,200 and U.S. Pat. No. 4,617,360. The catalyst ispreferably introduced only into the first polymerisation reactor, i.e.there is no introduction of fresh catalyst into the furtherpolymerisation reactor.

The cocatalyst utilised in the process is preferably an organoaluminiumcompound. Unhalogenated organoaluminium compounds of formula AlR₃ inwhich R represents an alkyl grouping having from 1 to 8 carbon atoms arepreferred. Particularly preferred are triethylaluminium andtriisobutylaluminium. The cocatalyst is introduced into the firstpolymerisation reactor. Fresh cocatalyst may also be introduced into thefurther reactor. The quantity of cocatalyst introduced into the firstreactor is in general at least 0.1×10⁻³ mole per litre of diluent. Itdoes not usually exceed 5×10⁻³ mole per litre of diluent. Any quantityof fresh cocatalyst introduced into the further reactor does not usuallyexceed 5×10⁻³ mole per litre of diluent.

In the preferred process of the invention using a Ziegler-Nattacatalyst, the polymerisation temperature is generally from 20 to 130° C.It is preferably at least 60° C. For preference, it does not exceed 115°C. The total pressure at which the process is effected is in generalfrom 0.1 MPa to 10 MPa. In the first polymerisation reactor, the totalpressure is preferably at least 2.5 MPa. Preferably, it does not exceed5 MPa. In the further polymerisation reactor, the total pressure ispreferably at least 1.3 MPa. Preferably, it does not exceed 4.3 MPa.

The period of polymerisation in the first reactor and in the furtherreactor is in general at least 20 minutes, preferably at least 30minutes. It does not usually exceed 5 hours, preferably not 3 hours.

In this process, a suspension comprising the resin of the invention iscollected at the outlet of the further polymerisation reactor. Thecomposition may be separated from the suspension by any known means.Usually, the suspension is subjected to a pressure expansion (finalexpansion) so as to eliminate the diluent, the ethylene, thealpha-olefin and any hydrogen from the composition.

This process makes it possible to obtain, with a good yield and with alow oligomers content, a composition having a very good compromisebetween mechanical properties and utilisation properties.

The compositions of the invention are well suited to the manufacture ofpipes and pipe fittings, particularly pipes for the conveying of fluidsunder pressure such as water and gas. The invention therefore alsoconcerns the use of a composition according to the invention for themanufacture of pipes or pipe fittings. Naturally, when they are used forthe molten forming of articles, and more particularly for themanufacture of pipes or pipe fittings, the compositions of the inventionmay be mixed with the usual additives for utilisation of polyolefins,such as stabilisers (antioxidant agents, anti-acids and/or anti-UVs),antistatic agents and utilisation agents (“processing aid”), andpigments. The invention therefore concerns also a mixture comprising acomposition according to the invention and at least one of the additivesdescribed above. Particularly preferred are mixtures comprising at least95%, preferably at least 97%, by weight of a composition according tothe invention and at least one of the additives described above. Themanufacture of pipes by extrusion of a composition according to theinvention is preferably carried out on an extrusion line comprising anextruder, a sizer and a drawing device. Extrusion is generally performedon an extruder of the single-screw type and at a temperature of 150 to230° C. The sizing of the pipes may be effected by the creation ofnegative pressure outside the pipe and/or by the creation of positivepressure inside the pipe.

Pipes manufactured by means of the compositions according to theinvention are characterised by:

good resistance to slow propagation of cracks (FNCT), reflected by arupture time, as measured by the method described in the standardundergoing preparation ISO/DIS 16770.2 (2001) (at 80° C., under a stressof 5.0 MPa (in an Arkopal N100 2% solution)), greater than 100 hours;

good resistance to rapid propagation of cracks (RCP), reflected by ahalting of crack propagation at an internal pressure generally equal toat least 10 bar, as measured at 0° C. on a pipe of diameter 110 mm andthickness 10 mm according to method S4 described in ISO standard F/DIS13477(1996);

good long-term pressure resistance which may enable them to beattributed a higher MRS rating than the MRS 10 rating according tostandard ISO/TR 9080.

The following examples are intended to illustrate the invention.

EXAMPLES 1-6 a) Preparation of the Catalyst

Magnesium diethylate was caused to react for 4 hours at 150° C. withtitanium tetrabutylate in quantities such that the molar ratio oftitanium to magnesium was equal to 2. Polymerisation grade hexane wasadded to the alcoholate mixture in order to obtain a solution containing250 g of mixture per litre of solution. Thereafter the reaction productthus obtained was chlorinated and precipitated by placing it in contactwith an ethylaluminium dichloride solution for 90 minutes at 45° C. Thecatalyst thus obtained, gathered from the suspension, comprised (% byweight):

Ti: 17; Cl: 41; Al: 2; Mg: 5.

b) Polymerisation

An ethylene polymer composition in suspension in isobutane wasmanufactured in two loop reactors connected in series, separated by adevice making it possible to effect pressure expansion continuously.

The catalyst as described in item a) of Example 1 was introducedcontinuously into the first loop reactor, and polymerisation of theethylene to form polymer (A) was effected in this medium. Said mediumcomprising polymer (A) in addition was drawn off continuously from saidreactor and was subjected to expansion (48° C., 0.6 MPa) so as toeliminate at least part of the hydrogen. The resulting medium at leastpartially degassed from hydrogen was then introduced continuously into asecond polymerisation reactor at the same time as ethylene, hexene,isobutane and hydrogen, and polymerisation of the ethylene and thehexene was effected in order to form the copolymer (B). The suspensioncomprising the polymer composition was drawn off continuously from thesecond reactor and this suspension was subjected to a final expansion soas to evaporate the isobutane and the reagents present (ethylene, hexeneand hydrogen) and recover the composition in the form of a powder thatwas subjected to drying in order to finish the degassing of theisobutane.

The other polymerisation conditions are specified in Table 1.

Compounding

To the resins of Examples 1 to 6 were added, per 100 parts ofpolyethylene composition, 0.35 parts by weight of antioxidant IRGANOX®B225, 0.075 parts by weight of zirconium stearate, 0.025 parts of weightof calcium stearate and 2.25 parts by weight of carbon black.

The resultant composition was extruded on a compounding devicecomprising a melting zone (single screw extruder, 90 mm screw diameter,24D length) and a homogenising zone (single screw extruder, 90 mmdiameter screw, 36D length) at a rate of 40 kg/hr and with a residencetime of 540 seconds. At the end of the compounding device, the resultingcompound was passed through a strand pelletiser and pellets of theresulting compound were recovered. TABLE 1 EXAMPLE 1 2 3 4 5 6 REACTOR 1C₂ (g/kg 18.8 19.8 21.1 20.3 20.3 20.5 diluent) H₂/C₂ mole 63.8 43.843.1 47.4 48.4 46.2 ratio T (° C.) 85 85 85 85 85 85 dwell time (h) 1.331.3 1.32 1.32 1.32 1.32 REACTOR 2 C₂ (g/kg 14.9 13.8 12.0 14.9 15.1 14.3diluent) H₂/C₂ mole 0.73 0.51 0.47 0.71 0.69 0.56 ratio C₆/C₂ mole 187.7177.6 183.2 143.7 128.1 147.0 ratio T (° C.) 75 75 75 75 75 75 dwelltime (h) 1.29 1.28 1.3 1.32 1.28 1.29

TABLE 2 EXAMPLE 1 2 3 4 5 6 Reactor 1 p1 (wt %) 48.9 53.1 52.7 49.0 49.049.0 MI₂ 27.4 7.5 10.8 8.9 8.3 9.3 (g/10 min)^(#) Density 973.7 972974.8 971.5 971.2 971 (kg/m³) Final granulated product MI₅ 0.57 0.580.45 0.68 0.57 0.44 (g/10 min) MI₂₁ 13.1 11.7 11.8 12.8 10.3 9.9 (g/10min) μ₂ 20 20.8 21.2 21 21.9 22.7 (1000 Pa · s) Mw/Mn 18.89 15.98 18.3717.84 15.56 19.61 Density 958.9 959.9 958.2 959.9 960.2 958.1 (kg/m³)*^(#)MI₂ measured with 8 mm length/1.0 mm internal diameter die, whichgives values approximately 19 times lower than an 8 mm/2.095 mm die*including pigmentMechanical Property TestsCreep Resistance

Creep resistance was evaluated on 50 mm SDR 17 pipes produced on KMand/or Battenfeld extruders according to ISO 1167. The pressure testsresults are on the basis of an MRS rating equal 10 MPa, according to thestandard IS09080.

Stress Crack Resistance

Stress crack resistance was evaluated on notch pipe tests, diameter 110mm, SDR 11, according to ISO13479.

Rapid Crack Propagation

Rapid crack propagation was determined at an internal pressure generallyequal to at least 10 bars, as measured at 0° C. on a pipe of diameter110 mm and thickness 10 mm according to method S4 described in IS013477.TABLE 3 EXAMPLE 1 2 3 4 5 6 Creep Pipe 1 - 12.4 MPa 142 190 138 142 337209 Pipe 2 - 12.4 MPa 152 200 121 184 418 198 Pipe 1 - 12.1 MPa 937 731538 879 1149 465 Pipe 2 - 12.1 MPa 1018 815 583 989 1160 459 Pipe 1 -5.5 MPa 167 230 208 897 3898 449 Pipe 2 - 5.5 MPa 140 277 277 1070 4129555 Pipe 1 - 5.0 MPa 6521 4820 — 4663 4201 >5820 Stress crackresistance - notched pipe test Pipe 1 - 4.6 MPa 3498 2402 7956 1537 15634100 Pipe 2 - 4.6 MPa 3470 2526 8712 1414 1602 4663 Rapid crackpropagation 0° C. >10 >10 >10 >10 >10 −5° C. >8 >10 >10 >10 >10 −10°C. >10 >10 4 >10 >10 −15° C. 3.50 3.50 3.50 >5It can be seen from the above table that all the resins pass theEuropean requirements (EN1555—EN12201—IS04427—IS04437) for creepresistance of a PE100 resin:

-   -   At least 100 hours at 12.4 MPa, 20° C.    -   At least 165 hours at 5.5 MPa, 80° C. without brittle failure    -   At least 1000 hours at 5 MPa, 80° C.        For stress crack resistance, all of the resins comfortably pass        the European requirements (EN1555—EN12201—IS04427—IS04437) for a        PE100 resin, which is a resistance time at least equal to 165        hours at 80° C., 5.5 MPa.

All the resins show good resistance to rapid crack propagation (RCP).

1-12. (canceled)
 13. Polyethylene resin having an MI₅ of from 0.40 to0.70 g/10 min, and comprising from 47 to 52 wt % of a low molecularweight polyethylene fraction, and from 48 to 53 wt % of a high molecularweight polyethylene fraction, wherein the high molecular weightpolyethylene fraction comprises a copolymer of ethylene and either1-hexene or 1-octene.
 14. Resin according to claim 13, wherein the lowmolecular weight polyethylene fraction comprises a polyethylene having adensity of at least 0.965 g/cm³ and an MI₂ of from 5 to 1000 g/10 min,and the high molecular weight polyethylene fraction has a density offrom 0.910 to 0.940 g/cm³ and an MI₅ of from 0.01 to 2 g/10 min. 15.Resin according to claim 13, characterised in that it comprises 48-51%by weight with respect to the total weight of the composition of theethylene polymer fraction (A).
 16. Resin according to claim 13,characterised in that it comprises 49-51% by weight with respect to thetotal weight of the composition of the ethylene polymer fraction (A).17. Resin according to claim 13, characterised in that it has a densityafter granulation and addition of additives of between 957 kg/m³ and 963kg/m³.
 18. Resin according to claim 13, characterised in that it has anMI₅ of from 0.45 to 0.65 g/10 min, preferably 0.50 to 0.60 g/10 min. 19.Resin according to claim 13, characterised in that it has a densityafter granulation and addition of additives between 959 kg/m³ and 961kg/m³, or a natural density of between 950 and 952 kg/m³.
 20. Resinaccording to claim 13, characterised in that the composition comprisesat least 0.2% mole % and at most 0.7 mole % of monomer units derivedfrom 1-hexene or 1-octene.
 21. Polyethylene resin having an MI₅ of from0.40 to 0.70 g/10 min, a density after granulation and addition ofadditives between 957 kg/m³ and 963 kg/m³, and a stress crack resistancetime of at least 165 hours at 80° C. and 5.5 MPa according to standardsEN1555—EN12201—IS04427—IS04437.
 22. Resin according to claim 21,comprising from 47 to 52 wt % of a low molecular weight polyethylenefraction, and from 48 to 53 wt % of a high molecular weight polyethylenefraction, wherein the low molecular weight polyethylene fractioncomprises a polyethylene having a density of at least 0.965 g/cm³ and anMI₂ of from 5 to 1000 g/10 min, and the high molecular weightpolyethylene fraction has a density of from 0.910 to 0.940 g/cm³ and anMI₅ of from 0.01 to 2 g/10 min.
 23. Resin according to claim 21,characterised in that it has an MI₅ of from 0.45 to 0.65 g/10 min,preferably 0.50 to 0.60 g/10 min.
 24. Resin according to claim 21,characterised in that it has a density after granulation and addition ofadditives between 959 kg/m³ and 961 kg/m³, or a natural density ofbetween 950 and 952 kg/m³.
 25. Resin according to claim 21,characterised in that the composition comprises at least 0.2% mole % andat most 0.7 mole % of monomer units derived from 1-hexene or 1-octene.26. Pipe or pipe fitting comprising a resin as defined in claim
 13. 27.Use of a resin as defined in claim 13 as a pipe or pipe fitting.